The thermal stability of organic porous low k, porous SiLK with a dielectric constant of 2.4, has been studied. Organic low k material SiLKTM, non-porous SiLK, with a dielectric constant 2.8 is used as a baseline for comparison. Each sample was subjected to annealing cycles, where each cycle was conducted in a vertical furnace for one hour in an N2 ambient. The annealing temperature was set at either 430°C or 450°C. After every alternate cycle, the film properties were measured and compared to the unannealed sample for changes in film shrinkage, refractive index, dielectric constant, roughness, breakdown voltage, pore size, hardness and Young's modulus. Changes in film properties were investigated and evaluated by using opti-probe, FTIR, XPS, AFM, mercury probe, nano-indentation, SEM and TEM techniques.